Origin of Comets

Subaru Approaches Origin of Comets ---
First Estimate of the Formation Temperature of Ammonia Ice in a Comet

Observations made with the High-Dispersion Spectrograph (HDS) of
Subaru Telescope have, for the first time, allowed astronomers to measure
the formation temperature of ammonia ice in a comet. The temperature of 28
+/- 2 Kelvin (about -245C or -410F) suggests that this comet, Comet LINEAR
(C/1999 S4), was formed between the orbits of Saturn and Uranus. These
observations provide us with not only direct evidence of the environment in
which the comet was born, but also establish brand new methods for probing
the origin of comets.

Comet LINEAR was discovered in 1999 by the Lincoln Near Earth
Asteroid Research project (LINEAR), operated by the MIT Lincoln Laboratory.
Figure 1 shows two images of Comet LINEAR obtained by Subaru Telescope in
2000 (see Latest News on July 24th, 2000). A team of researchers from the
National Astronomical Observatory of Japan, the HDS group, and the Gunma
Astronomical Observatory made spectroscopic observations of Comet LINEAR on
July 5th, 2000, during the commissioning phase of HDS, when the comet was
bright.

The team concentrated on the emission lines produced when NH2
molecules which have been previously excited, lose some energy and emit
light at a series of characteristic wavelengths (Figure 2). Previous studies
indicate that NH2, which consists of one nitrogen and two hydrogen atoms, is
produced when the powerful Solar UV rays free a hydrogen atom from the
ammonia (NH3) gas which is constantly boiling off the comet. The emission
lines of the NH2 molecules should therefore contain information on their
parent ammonia molecules.

Molecules like NH2 and NH3 which contain two or three hydrogen
atoms are classified as either "ortho" or "para", depending on whether the
quantum mechanical spins of the hydrogen atoms are aligned or not. The
ortho-to-para ratio strongly depends on the physical environment, and would
have been preserved when the molecules were confined into the icy cometary
nuclei. The observed ratio can therefore reveal the temperature at the time
the ice was formed.

Molecules in the ortho and para states emit radiation at
wavelengths which are very close together, but subtly different due to the
differences in alignment between the spins of the hydrogen atoms. The
resolving power of HDS is high enough to separate these lines and determine
how much light is being emitted by molecules in the ortho and para states.
Using code written by Mr. Hideyo Kawakita of the Gunma Astronomical
Observatory, the strengths of the emission lines from NH2 could be modeled
and compared with the observations to determine the ratio of ortho to para
molecules in Comet LINEAR. Furthermore, the team investigated the
ortho-to-para ratio of the parent NH3 molecules and estimated that the
formation temperature of the ammonia ice to be 28 +/- 2 Kelvin, which
suggests that Comet LINEAR was formed between the orbits of Saturn and
Uranus in the primordial Solar System nebula.

Until now, the formation temperature had only been determined for
water ice in comets, and this is the first time that it has been measured
for another molecule. Dr. Jun-ichi Watanabe of the National Astronomical
Observatory of Japan, and a member of the team who performed this research,
says "The brand new methods using NH2 molecules have great potential for
studying the origin of comets. I have a high expectation for future results
obtained by these methods, especially for short-period comets which are
thought to have a different origin from long-period comets such as Comet
LINEAR."